Freezing and Thawing

Abstract

Freezing is an important operation in food preservation, for it involves millions of tons of food per year (Pierce 2002). During freezing, ice formation begins at a temperature Tif, characteristic to each type of foodstuff, and continues over a wide temperature range. The water phase change brings about an important and continuous variation of the characteristic physical properties of the phenomenon (ρ, Cp, k). It is important to take into account that this phase change does not occur at a constant temperature but instead takes place as a dynamic transformation over a range of temperatures, which affects the whole freezing process. According to its definition (International Institute of Refrigeration 1972), effective freezing time is the time required to lower the temperature of the product from its initial temperature to a given temperature at its thermal center (generally −10°C or −18°C). When a foodstuff is industrially frozen, the process has to be controlled closely, since food reaching the storage stage partially frozen means an increase in energy consumption. Yet, it is economically unfavorable to refrigerate the product to a lower temperature than the storage one. Thawing is another important food operation related to industrial processed foods. Thus, thawing times must be calculated properly to control this stage in a food processing plant. Therefore, adequate methods for predicting freezing and thawing times should be available to design and evaluate plants for processed foods. In this sense, one of the main interests of design engineers and equipment users is to be able to count on simple and accurate prediction methods for the simulation of the process they are dealing with, mainly for the calculation of process times as a function of material characteristics and operating conditions.